Te Mihi Power Station is a two-unit 166-MW geothermal plant currently undergoing commissioning on New Zealand’s North Island. It replaces the Wairakei Power Station constructed in 1958—but with a much smaller environmental footprint. The double flash technology selected produces ~25% more power from the same amount of geothermal fluid that is currently used at Wairakei. For its continuing commitment to renewable geothermal energy, Contact Energy Ltd.’s Te Mihi Power Station is the winner of POWER’s 2013 Marmaduke Award for excellence in power plant problem-solving. The award is named for Marmaduke Surfaceblow, the fictional marine engineer and plant troubleshooter par excellence.

Contact Energy Ltd. (Contact) is one of New Zealand’s leading developers of sustainable power generation systems, with a diverse portfolio of geothermal, natural gas, wind, and hydroelectric assets. In terms of revenue, Contact is one of five large New Zealand power companies. Contact owns and operates 10 plants located throughout the country, producing ~25% of New Zealand’s electricity demand. Four of its facilities are geothermal plants located in the Central North Island.

In early 2007, Contact announced plans to invest up to $1 billion in the construction of new geothermal plants in the Taupo region, located near the center of the North Island. (All amounts in US$; US$1 = NZ$1.28 at press time.) The latest addition to Contact’s renewable portfolio is the two-unit 166-MW (159-MW net) Te Mihi Power Station (Te Mihi).

Contact CEO Dennis Barnes says its investment in Te Mihi reflects the company’s view that geothermal is New Zealand’s most cost-effective new baseload generation. Barnes identified the importance of Te Mihi to ratepayers when he said, “The additional 114 megawatts is expected to be required by the market by 2013 as economic growth resumes and will also contribute to lowering Contact’s average cost of generation.” The total cost of Te Mihi is estimated to be close to $623 million. A second project at Tauhara is in the development pipeline, with other projects seeking permits or in the reservoir exploration phase.

To develop Te Mihi, Contact engaged the McConnell Dowell Constructors Ltd., SNC-Lavalin, and Parsons Brinckerhoff New Zealand joint venture (MSP JV) to build Te Mihi. The engineering, procurement, and construction (EPC) contract was signed with MSP JV in February 2011 for two 83-MW geothermal power units to be constructed 5 kilometers (km) from the existing Wairakei geothermal power station.

Long-Term Investment

Taupo is the center of volcanic and geothermal activity on the North Island (Taupo Volcanic Zone), and Lake Taupo is a popular tourist destination. Taupo is also the region where geothermal energy has been the principal source of electricity generation for the North Island since the 1950s. Today, geothermal energy satisfies almost 15% of the entire country’s electricity needs. All of the country’s geothermal plants are located on the North Island. Hydroelectric power supplies about 98% of total South Island demand. A single HVDC Inter-Island transmission line with a capacity of 1,200 MW (by the end of 2013) interconnects the two islands.

The oldest geothermal plant in the Taupo region is the 66-MW Wairakei A Power Station, constructed in 1958, the first of its type in the world to use wet steam in a single flash process to produce electricity. The plant’s single flash system has reliably served residents, but increasing maintenance costs and emerging environmental issues have shown the plant was nearing the end of its useful life. However, the Wairakei steam field is predicted to be able to supply steam for electricity generation for many more decades.

Wairakei uses once-through river cooling of its condensers and discharges steam condensate into the Waikato River, which flows down from Lake Taupo. At present, the Wairakei power station uses a cooling system that relies on drawing water from the Waikato River, mixing it with steam condensate that has been used to power the turbines, and then discharging that stream of cooling water back to the river. The result has been warmer water and the deposit of geothermal trace elements in the river water.

Conversion from once-through cooling to an evaporative system plus treatment for trace elements in the water discharge was not a economical solution, so development of Te Mihi was hastened. A newly built bioreactor now removes hydrogen sulfide from Wairakei’s cooling water before it is returned to the river.

Te Mihi is a staged replacement of the existing 157-MW Wairakei A and B power stations, although the 16-MW binary system added in 2005 that uses hot separated geothermal fluid will continue to operate. The net increase from the combined Te Mihi and binary portion of the Wairakei station is 114 MW of firm baseload capacity (Figure 1).

1. Global leader. Geothermal energy has produced electricity for New Zealand for more than 50 years. The 166-MW Te Mihi Power Station, developed by Contact Energy, is the latest addition to its renewable energy portfolio. The piping to the far right carries the low-pressure steam andintermediate-pressure geothermal water that is flashed to steam in a low-pressure flash system and sent to each of the two steam turbines, located in the main turbine hall (large blue building). To the left of the turbine hall are the two transformer bays and three power distribution center buildings. On the far left is the administration building and adjoining workshops. The site has
the added bonus of ready access to the 220 kV electricity transmission grid. Courtesy: MSP JV

Managing the Resource

The geothermal resource used to produce the motive steam originates with the Wairakei-Tauhara geothermal system, which contains pressurized water at a temperature of ~250C (482F) at a depth of about 2.5 km. The geothermal fluid production begins with rainwater that percolates through the volcanic rock until it is heated and pressurized by volcanic activity deep underground. After producing electricity at Te Mihi, the warm geothermal fluid is conserved by reinjection back into the Wairakei-Tauhara geothermal reservoir. Another disposal option available to Te Mihi for the clean condensate (~10% of the reinjection fluid) is farm spray irrigation, though this option is not currently utilized.

Steam for the Te Mihi Power Station is drawn from geothermal fluid in the Wairakei geothermal field from two production areas (Figure 2):

Western Borefield. About 40% of Wairakei Power Station’s steam currently comes from the Western Borefield. It consists of about 30 wells drilled to a depth of about 600 m characterized by the production of a two-phase fluid (steam/water mixture).

Te Mihi Borefield. Approximately 60% of Wairakei Power Station steam currently comes from the Te Mihi area, located about 5 km west of the existing Wairakei Power Station. This borefield produces dry steam from a shallow (300 m to 400 m) high-pressure (10 bar) steam zone, and two-phase fluid from a large reservoir of hot water below this at a depth of close to 2,500 m. The Te Mihi Borefield also provides steam for the Poihipi Road Station, another geothermal plant in the area.

2. Drilling for energy. The Te Mihi Power Station is powered by high-pressure, high-temperature
geothermal fluid that is brought to the surface through a series of wells that are up to 2.5-km deep.
The fluid flashes tosteam as itrises in the well. The piping carries the two-phase fluid to the
plant for separation. Courtesy: MSP JV

The new Te Mihi Borefield is closer to the Te Mihi plant to minimize steam transmission losses. It is also the deeper liquid resource that will continue to be developed to provide steam for Wairakei and Te Mihi Power Stations for the long term.

Of key importance to the plant design is that the Te Mihi plant site is at higher elevation (relative to the existing Wairakei Power Station) because this reduces the fluid pumping requirements for the disposal of separated hot water and condensate to the injection wells. At the same time, the plant site is below the intermediate pressure (IP) separation plant elevation, thereby allowing gravity feed of LP geothermal water to the low-pressure (LP) flash vessels at the station.

Flash Steam Process

The advanced “double flash” technology used on the Te Mihi project produces ~20% to 25% more electricity than the same amount of geothermal fluid consumed by Wairakei. In addition, the plant design uses an evaporative cooling system and eliminates the condensate discharges to the Waikato River, thereby reducing the plant’s environmental footprint in the region.

The design of the dual flash system uses steam released from the geothermal brine at two pressure levels. As the high-pressure liquid found at depth rises to the surface, the pressure decreases and the water flashes into a two-phase (80% water and 20% steam) mixture. At centralized separators, geothermal fluid is separated into IP steam and brine by the separated geothermal water (SGW) system. Both are piped to the power station, where the separated brine is flashed in the low-pressure SGW system to generate LP steam, which is injected into the steam turbine (Figure 3). (See the online version of this story for a link to a process drawing.)

3. Separate the steam. The two-phase mixture is separated into steam and water. The intermediate-pressure geothermal water is flashed a second time to produce low-pressure steam. The Unit 1 IP and LP steam scrubbers are shown. Courtesy: MSP JV

The Te Mihi Project inked a technology agreement that represents the re-emergence of Toshiba as a steam turbine technology provider for flash-type geothermal projects. Toshiba Corp., through its Australian subsidiary, Toshiba International Corp. Pty. Ltd., supplied two sets of 83-MW-class geothermal turbines, generators, and condensers, plus support services to the project. Despite the impact of the devastating March 2011 earthquake and tsunami in Japan, Toshiba met its initial delivery commitments to the project.

The double flash technology provides steam at two different pressure levels for introduction into a dual-admission steam turbine. The IP and LP steam is expanded through the steam turbine to drive the generator. The exhaust steam is condensed using evaporative cooling towers. The liquids (~100F) collected by the SGW systems are recycled by reinjection back into the geothermal reservoir. Unique to geothermal plants, some noncondensable gases (NCGs, such as CO2 and H2S) are dissolved in the geothermal water and remain in the steam. The NCGs are removed from the condensers by gas extractors and discharged to the atmosphere.

The power station facility also incorporates emergency steam vents to ensure that the steam pipelines are not overpressurized, which could cause problems with downstream generating equipment, such as the steam turbines and other vessels. These vents, arranged on the incoming IP and LP steam mains and a common LP vent adjacent to the LP flash vessel, incorporate automatic vent valve control systems and steam vent silencers.

Te Mihi is designed to be operated remotely from Wairakeim, but it does incorporate a small control room and staff facilities for on-site operation and maintenance. Roving operators visit Te Mihi regularly to check the status of equipment and perform on-site chemistry testing.

Shaky Start, Strong Finish

Contact’s Te Mihi power station has negotiated many twists and turns on its route to completion, starting with the rigorous consenting process. As the power station site was classed as rural, particular importance was attached to the effect of noise, visual impact, and traffic management during construction and operation. Stringent noise level limits were applied and extensive mitigation measures were put in place to achieve a compliant design. The site’s visual impact was lessened by restricting building heights, carefully selected main structure colors, and with plantings.

The traditional consent application in New Zealand, via a district or regional council, can be a lengthy process. Given the project’s significance for the New Zealand economy, Contact expedited the process by applying to the minister for the environment to designate the project as a “project of national importnace,” allowing him to exercise his right to “call in” the consent application. This meant that a Board of Inquiry selected by the minister rather than a local council would consider the application. Following a prompt Board of Inquiry process, consents were granted in 2008. Te Mihi was the first major power project that used the call-in process that is part of the Resource Management Act to expedite important projects.

Once consented, the development phase of the Te Mihi project encountered a number of challenges. For example, the global financial crisis led to a decision to defer the project by a year. During this hiatus, Contact completed additional work to refine the scope prior to further competitive bidding.

The final EPC procurement process restarted in 2010 and, on Feb. 22, 2011, an EPC contract was signed with MSP JV—on the same day that the devastating Christchurch earthquake rattled the South Island. The MSP JV readjusted work schedules and resources to regain project design and construction schedule losses incurred due to the earthquake.

The construction team mobilized on site in January 2011, and the design office was established in Auckland. Earth works commenced in April 2011 by carving an access road and a flat construction platform out of rolling farm land bounded with faults and sink holes, and implementing erosion and sediment controls. Some 500,000 m3 of earthworks was eventually completed. Piles were installed beginning in August 2011, followed by the start of installing 4,000 tonnes of structural steel during the period of March through October 2012. Mechanical and piping work began in January 2012, followed by electrical work in August 2012.

As this article was being prepared (early July 2013), commissioning work on both units was in full swing and will likely be completed by the time you are reading this article. Performance testing for both units is the next project milestone, followed by a one-month reliability run prior to commercial operation.

From its inception, Te Mihi incorporated environmental stewardship into every phase of the project. As an example, the Te Mihi design increased the number of reinjection wells added to the Wairakei steam field so that all geothermal fluid can be reinjected back into the ground rather than being discharged into the Waikato River. Very accurate pH control is essential for successfully reinjecting the separated geothermal water. Working with Contact Energy and key suppliers, the MSP JV has designed and built a unique acid injection system on a scale that has never been achieved with any prior geothermal project (Figure 4).

4. Acid dosing system. A unique feature of Te Mihi is its novel acidifying of the reinjection brine. The caustic LP separated geothermal water is well-known for its propensity to scale reinjection pipes, wells, and other equipment, causing lengthy plant shutdowns for repairs and clean-out. By acidifying the fluid, that system reliability problem was eliminated. Courtesy: MSP JV

5. The team. More than 2,500 staff members total have worked on site during the past two years, with numbers peaking at around 550. Courtesy: MSP JV

Geothermal energy is as renewable as that derived from the sun and wind, except that it has the added benefit of providing base-load power generation. On the North Island of New Zealand, geothermal energy has served customers for over 50 years. Te Mihi, built in the same tradition, will undoubtedly continue that tradition for another half-century. From the staff of POWER, congratulations to Contact Energy and the MSP Joint Venture for a job well done (Figure 5). (For more details, watch this video of the project.)

— Dr. Robert Peltier, PE is POWER’s editor-in-chief.

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